Renewed sonar searches and trawling has begun around the crash site of the Irish Coast Guard helicopter that went down in the Atlantic

Rescue 116 crashed off Co Mayo at 12.46am on March 14 with four crew on board after it struck Blackrock Island 12 miles off the coast of Co Mayo.

The bodies of winchmen Paul Ormsby and Ciaran Smith were never recovered despite weeks of intensive seabed, surface and shore searches.

Also on board were Captain Dara Fitzpatrick, the commander of the flight who was pulled from the sea in the hours after the crash, and Captain Mark Duffy, the co-pilot whose body was taken from the cockpit 12 days later by Navy divers.

With calmer summer seas and swells in the area, a Donegal based fishing boat, the MFV Westbound, has been fitted with specially designed nets as part of the operation. The boat is also running sonar systems which it normally uses for deep sea trawling.

In its initial work it has scoured the seas around Blackrock and Inishkea and Duvillaun islands a few miles off Mullet Head.

Superintendent Tony Healy, based at Bellmullet, said if any areas of interest show up during the seabed mapping then robot cameras can be deployed next weekend from the Irish Lights Vessel Granuaile.

The trawler has two specially made nets, one for covering rocky seabeds and another for work on sand. They are expected to be deployed on areas around the island which could not be dived on and then examined in detail.

"This is always something that we wanted to do," Supt Healy said.

"But to do it we needed calmer seas and to get a steady platform that was not available to us before. At this time of year we have better weather windows."

The sonar work started on Sunday and it is hoped the Killybegs trawler will work in the area for the rest of the week.

Some of the waters around the island and crash site are 60-70m (197ft - 230ft) deep, Supt Healy added.

An inquiry is ongoing to determine how Rescue 116 went down.

Ireland's Air Accident Investigation Unit (AAIU) has established that the Sikorksy S-92's internal warning system did not include the 80m (262ft) high rocky outcrop and its working lighthouse in its database.

The search and rescue helicopter crashed after hitting Blackrock Island while coming back from supporting a mission for an injured trawlerman out in the Atlantic.